Once there were three friends, who were known of their intelligence in solving logical problems. People from near and far arrived to test their skills. Once an old woman came to see them. She had three large black balls and two red balls, and she asked the friends to solve a problem involving these.
She asked our friends to sit in a circle so that each of them could see behind the two others. Then she showed the balls to the friends and said: "Now I'll put one of these balls behind each of you so that you will not know which ball I put there, but you can see the two other balls. In five minutes one of you should be able to figure out, what colour is the ball behind him."
Then she put a black ball behind each of them and took the two red balls away. After four minutes, one of the friends said: "I have a black ball behind me." How could he know it?
Published in Njet Problem! collection 1996-05-21.